1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for measuring a bloodstream volume.
2. Description of the Related Art
An instantaneous bloodstream volume (to be merely referred to as a bloodstream volume hereinafter) is obtained by integrating an instantaneous bloodstream velocity for the section of a blood vessel. Since the bloodstream velocity in a blood vessel is not generally uniform (it is fast at the central portion of the blood vessel and slow near the wall of the blood vessel due to the viscosity of the blood), to correctly obtain an instantaneous bloodstream volume, the velocity profile across the axis of the blood vessel must be obtained. In practice, a bloodstream volume is calculated based on a bloodstream velocity of a center of the cross-section of the blood vessel and the cross-section. A method of obtaining the velocity profile along a profile axis of the cross-section of a blood vessel and calculating the instantaneous bloodstream volume based on the velocity profile is usually employed under an assumption that the section of a blood vessel is circular. This calculation of the instantaneous bloodstream volume includes the following various factors that cause errors.
(1) As the blood vessel moves in accordance with the respiration and motion of the heart, the profile axis and the cross-sectional position change relative to each other.
(2) As the range (dynamic range) of detectable minimum and maximum bloodstream velocities is widely set in accordance with the maximum velocity in one heartbeat in order to avoid "aliasing" the velocity resolution at a low velocity is degraded.
(3) A signal level can become zero mainly due to scattering noise, and accordingly the apparent bloodstream velocity becomes zero even when a bloodstream exists.
(4) Since a reverse bloodstream component is added as a negative value, a one-heartbeat ejection volume expressing the total of forward bloodstream components of one heartbeat cycle cannot be obtained.
(5) Although a blood vessel diameter is increased and decreased in accordance with the motion of the heart, the same blood vessel diameter is used in practice throughout all the time phases.
(6) The precision of the one-heartbeat ejection volume is determined by the time and space resolutions. Since the time and space resolutions are inversely proportional to each other, an increase in precision is limited.
(7) No means is available for the observer to confirm whether or not an obtained instantaneous bloodstream volume includes an error.